1. Field of the Invention
The present invention relates to an optical transmission apparatus for an optical communication system, and more particularly to a polarization-shaped duobinary optical transmission apparatus using a polarization-shaped duobinary optical transmission scheme.
2. Description of the Related Art
In general, a DWDM (Dense Wavelength Division Multiplexing) optical transmission system can transmit an optical signal having multiple channels with different wavelengths through a single optical fiber. Such DWDM systems can also transmit the optical signal regardless of transmission speed. Because of this, such DWDM systems are widely used in ultra-high speed Internet networks. In this regard, systems using the DWDM technology can transmit more than a hundred channels through a single optical fiber.
However, the enlargement of transmission capacity is restricted due to severe interference and distortion between channels if the channel distance is less than 50 GHz when a optical intensity is modulated using the conventional non-return-to-zero (NRZ) method. This is due to a rapid increase of data traffic and a request for high-speed transmission of data of more than 40 Gbps. Transmission distance is restricted in high-speed transmission of more than 10 Gbps since a direct current (DC) frequency component of a conventional binary NRZ transmission signal and a high frequency component spread during modulation cause non-linearity and dispersion when the binary NRZ transmission signal propagates in an optical fiber medium.
Optical duobinary technology has been highlighted as an optical transmission technology capable of overcoming restriction of transmission distance due to chromatic dispersion. A main advantage of the duobinary transmission is that the transmission spectrum is reduced in comparison to the general binary transmission. In a dispersion restriction system, the transmission distance is in inverse proportion to the square of the transmission spectrum bandwidth. This means that, when the transmission spectrum is reduced by ½, the transmission distance increases four times. Furthermore, since a carrier frequency is suppressed in a duobinary transmission spectrum, it is possible to relax the restriction of an optical power output caused by Brillouin scattering excited in the optical fiber.
In this regard, polarization-shaped duobinary modulation schemes have a characteristic which is strong with respect to narrow signal bandwidths and nonlinear distortion of optical fibers. An optical signal having intensities of ‘1’ level and ‘0’ level as shown in FIG. 1, in which ‘1’ level represents an optical signal having two orthogonal polarization-shaped intensities (1⊥ or 1∥) and ‘0’ level represents an optical signal having an intensity of zero, an ideal optical intensity. However, since the optical intensity of polarization-shaped duobinary is characterized by the binary, ‘1’ and ‘0’, its receiver is employed without modifying a receiver of a typical optical transmission system having OOK (on-off keying) mode.
A characteristic of the polarization-shaped duobinary signal is an narrow bandwidth that is powerful against nonlinear distortion of optical fiber signal. The bandwidth of the polarization-shaped duobinary signal is narrower than that of a typical OOK (on-off keying) signal or AM-PSK duobinary signal so that the polarization-shaped duobinary contributes to improving the frequency efficiency of DWDM optical transmission systems. Also, since the polarization-shaped duobinary signal include ‘1’ level bits which comprises orthogonal polarization-shaped bits, the polarization-shaped duobinary signal is different from the OOK (on-off keying) signal or AM-PSK duobinary signal, and so are powerful against an nonlinear distortion of optical fiber.
FIG. 2, illustrates a structure of a conventional duobinary optical transmitter. First, a binary NRZ input data signal is input to and coded by a differential encoder 1. Generally, since an optical modulator signal and an input received signal are different, a duobinary transmitter encodes the input data signal using a differential encoder 1 in a transmission section so that the received signal and transmission data are the same. The encoded signal is split into two, one is directly input to a first duobinary filter 2, the other is input to a second duobinary filter 4 through an inverter 3. Duobinary filters 2 and 4 are narrow band filters having a bandwidth corresponding to about ¼ transmission speed (transmission speed by 0.25). The binary signal input to those filters is converted into a ternary signal, ‘+1’, ‘0’, and ‘−1’ level signal,. Half-wave rectifiers 5 and 8 pass a positive signal and convert a negative signal into zero. The ternary signal passing through the half-wave rectifier 5 is a binary signal having ‘+1’ and ‘0’ level, because ‘−1’ level converts into ‘0’ level. An output signal from the half-wave rectifier 5 is input to an optical intensity modulator 7 through an optical modulator driver 6. An inverted signal by an inverter 3 is input to other optical intensity modulator 10 through the half-wave rectifier 8 and an optical modulator driver 9. A light having continuous wave from a laser diode is split into two polarization beams having orthogonality to each other by an orthogonal polarization beam splitter 12. Each of two polarization beams is input to respective optical intensity modulators 7 and 10 and is synthesized by a polarization-maintaining coupler 13.
However, such conventional duobinary optical transmitters have several shortcomings. First, since the conventional duobinary optical transmitter has a (up and down) symmetrical structure, as shown in FIG. 1, duobinary filters, half-rectifiers, optical modulator drivers, and optical intensity modulators, except a differential encoder, an inverter and a polarization beam splitter, are allocated by two in the conventional duobinary optical transmitter. Therefore, the conventional duobinary optical transmitter requires a plurality of electric elements so that its implementation cost is very high. Secondly, since the (up and down) symmetrical structure of the conventional duobinary optical transmitter requires the symmetrical structure of electric/optical elements, reliability and reproducibility of its implementation are degraded. Lastly, because of deterioration of half-rectifiers in the conventional optical transmitter, such as that shown FIG. 2, ‘1’ bit polarization orthogonality dwindles, and as a result, the tolerance of an optical fiber nonlinear characteristic weakens.